Cofilin 1 (cofilin, non-muscle isoform 18 kDa phosphoprotein) protein is a cytoskeleton binding protein belonging to the ADF/COFILIN family and is one of the most highly conserved proteins among mammals. The amino acid sequence of the cofilin 1 protein has been determined for humans, mice, rats, chimpanzees, cattle, dogs, and the like, and is known to show 98% or higher identity among these species. It has been revealed that the cofilin 1 protein is involved in various biological phenomena including regulation of cell morphology and motility (Non-patent Literature 1), cytokinesis (Non-patent Literature 2), endocytosis (Non-patent Literature 3), and the like. Furthermore, the protein is also expressed at high levels in cancer tissues and cancer cells from lung cancer (Non-patent Literature 4), cancer of pancreas (Non-patent Literature 5), breast cancer (Non-patent Literature 6), ovarian cancer (Non-patent Literature 7) or hepatic cancer (Non-patent Literature 8). Involvement of the cofilin 1 protein in the progression of cancer has also been reported (Non-patent Literature 9).